chore: re-add $mat-font-family

Author: crisbeto

src/lib/core/style/_variables.scss

@@ -4,6 +4,9 @@
 $mat-xsmall: 'max-width: 600px';
 $mat-small: 'max-width: 960px';
 
+// TODO(crisbeto): this isn't being used anywhere within Material. keeping for backwards compat.
+$mat-font-family: Roboto, 'Helvetica Neue', sans-serif !default;
+
 // TODO: Revisit all z-indices before beta
 // z-index master list